Jason Leite adds spices to his fasting meal which includes chicken, vegetables and nuts, which he prepares at his home in Mountain House, Calif. on Thursday June 5, 2014. Leite lost 60 pounds in one year by intermittent fasting, a dieting trend that was recently popularized in a book. He is now eating a healthy amount of food one day, then cutting down to about 500 calories on the other days.

Jason Leite adds spices to his fasting meal which includes chicken,...

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Jason Leite adds a drizzle of Mexican crema to his fasting meal which includes chicken, vegetables and nuts, prepared at his home in Mountain House, Calif. on Thursday June 5, 2014. Leite lost 60 pounds in one year by intermittent fasting, a dieting trend that was recently popularized in a book. He is now eating a healthy amount of food one day, then cutting down to about 500 calories on the other days.

Jason Leite adds a dash of whipped cream to his protein shake to go along with his fasting meal which includes chicken, vegetables and nuts, prepared at his home in Mountain House, Calif. on Thursday June 5, 2014. Leite lost 60 pounds in one year by intermittent fasting, a dieting trend that was recently popularized in a book. He is now eating a healthy amount of food one day, then cutting down to about 500 calories on the other days.

Jason Leite adds a dash of whipped cream to his protein shake to go...

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Jason Leite with his fasting meal which includes chicken, vegetables and nuts along with a coconut milk protein shake, which he prepared at his home in Mountain House, Calif. on Thursday June 5, 2014. Leite lost 60 pounds in one year by intermittent fasting, a dieting trend that was recently popularized in a book. He is now eating a healthy amount of food one day, then cutting down to about 500 calories on the other days.

Jason Leite displays his fasting meal which includes chicken, vegetables and nuts. The meal he prepared at his home in Mountain House, Calif. on Thursday June 5, 2014. Leite lost 60 pounds in one year by intermittent fasting, a dieting trend that was recently popularized in a book. He is now eating a healthy amount of food one day, then cutting down to about 500 calories on the other days.

Jason Leite creates his protein shake to go along with his fasting meal which includes chicken, vegetables and nuts, prepared at his home in Mountain House, Calif. on Thursday June 5, 2014. Leite lost 60 pounds in one year by intermittent fasting, a dieting trend that was recently popularized in a book. He is now eating a healthy amount of food one day, then cutting down to about 500 calories on the other days.

Jason Leite displays his fasting meal which includes chicken, vegetables and nuts along with a coconut milk protein shake, prepared at his home in Mountain House, Calif. on Thursday June 5, 2014. Leite lost 60 pounds in one year by intermittent fasting, a dieting trend that was recently popularized in a book. He is now eating a healthy amount of food one day, then cutting down to about 500 calories on the other days.

Jason Leite with his fasting meal which includes chicken, vegetables and nuts along with a coconut milk protein shake, prepared at his home in Mountain House, Calif. on Thursday June 5, 2014. Leite lost 60 pounds in one year by intermittent fasting, a dieting trend that was recently popularized in a book. He is now eating a healthy amount of food one day, then cutting down to about 500 calories on the other days.

With about a third of adults tipping their scales into obesity, it's not surprising to see one diet after another sweep America off its feet: Move over Atkins and South Beach, now there's Paleo, cleanses and fasting.

Though nothing new, intermittent fasting has gained traction recently with "The FastDiet." In their best-selling book, which outlines one of a few popular fasting regimens, journalists Dr. Michael Mosley and Mimi Spencer advocate people slim down by eating normally five days a week, but cut back to a quarter of daily recommendations the other two days.

For some, it's just easier to abide calorie cutting a few days out of the week than to count calories every day, the authors say. But they also hype a number of potential health benefits from intermittent fasting that include reducing the risk of diabetes, heart disease and cancer.

Generally speaking, losing weight has been shown to reduce diabetes and heart disease risk factors, and as for cancer, much of the research has been in mice.

"In terms of dieting, intermittent fasting is another form of self-discipline," said Jo Ann Hattner, a registered dietitian and nutrition consultant to the Stanford University School of Medicine. "The bottom line is that Americans are gaining weight because they eat too many calories, and, like passing on seconds at dinner, it's a way to teach the body it doesn't need so much food."

But every person's metabolism is different, so some people tolerate intermittent fasting better than others, Hattner said. Those who try it, she added, should do so in a controlled environment with the guidance of a health professional.

Health risks

"You don't want to have some kind of very low drop in your blood sugar and you happen to be driving," she said. "There are some metabolic consequences."

Hattner said the usual suspects - pregnant and nursing women, diabetics and the elderly - shouldn't try the diet.

Jayson Leite, a personal fitness and nutrition coach from Mountain House (San Joaquin County) swears by the diet. He lost 60 pounds in a year of intermittent fasting combined with exercise.

But, Leite said, fasting isn't a magic formula for weight loss; it's very simple: At a calorie deficit, people lose weight. At a calorie surplus, they gain.

"At the end of the day, it's calories in, calories out," he said.

In 2009, after he was diagnosed as pre-diabetic, Leite discovered the Leangains intermittent fasting system, which prescribes 16 hours of fasting per day and feeding over eight hours, plus exercise. At 5 feet, 6 inches and 200 pounds, nothing he'd tried up to that point had worked.

At first, Leite said it was a difficult adjustment to make, not eating as much as he was used to for periods of time.He would sneak food, then send himself text messages telling himself not to do that again. But he found it more manageable than other diets.

There was an end in sight by fasting 16 hours a day instead of continuous dieting, he concluded. Like those whom researchers had observed in controlled weight-loss studies, Leite adapted within two weeks.

Now, he said he's been on some form of intermittent fasting ever since, is down to 140 pounds and has kept the weight off.

In 2012, Leite, who had worked in the mortgage business, changed his lifestyle and became a certified personal fitness and nutrition coach. He runs Look Fitness and Nutrition Management, and encourages his clients to try intermittent fasting. He's even converted some of the people who thought he was crazy for starting the diet.

"It's just like anything, once you get a taste of something, you want more," he said.

Though it's not for everyone, Leite said there are many ways to fast intermittently, and it can be adapted to fit different lifestyles. It's not realistic, for example, to expect his college athlete clients to eat 500 calories some days, or mind their diets on the weekends as they're trying to get lean or bulk up. So he has them on a Leangains-like system Sundays through Thursdays.

Part of the rationale for "The FastDiet" and others like it is that intermittent fasting is natural and even has benefits related to the way humans evolved, often without a meal ticket except their own hunting and gathering skills.

Hellerstein, the UC Berkeley professor, called the benefits beyond weight loss in humans untested and mostly anecdotal. Much of the research on fasting has been in mice: In a 1930s study, calorie-deprived mice lived up to 40 percent longer than their well-fed counterparts. On a calorie-restricted diet, studies have shown animals have reduced risk of cancer, heart disease and diabetes.

With intermittent fasting, "you're tricking the body into thinking its going through a deprivation, starvation period but it really isn't," Hellerstein said, calling starvation a beautifully orchestrated process. Eventually, in a fasting state, the body switches from burning proteins to burning fats to fend off starvation. Cell division in mice slows as the body tries to expend less energy, which also cuts cancer risk.

"I still like it, but to me there's a big problem. And that is that mice and rats have a metabolism rate that is faster than people," Hellerstein said.

But human studies on the benefits of fasting beyond weight loss could prove difficult. Though there are groups of people who fast intermittently and could be studied, it's unclear how long they would have to do it to get the same benefits as mice, Hellerstein said. It's also daunting for people to face the prospect of being hungry and tired for the rest of their lives, even if they live to be 130.

Another diet

Hellerstein thinks that in terms of weight loss, it's just another way to diet.

Leite is interested in the potential longevity and disease-prevention aspects of intermittent fasting, but to him, it's mostly about keeping up the lifestyle that helped him get fit.

And if you ask him, it's worked. A while back, he said, if you checked his ID, you wouldn't believe he's the same person pictured for all the weight he's lost.

"I've been questioned by law enforcement because I had my old ID on me," he said.